Float controls: compute: fix for strict 16bit storage validation

Float controls: compute: fix for strict 16bit storage validation

Emit definitions of float constants only if those constants
are used in the rest of the shader.  In the case of 16bit
float this saves us from having to also declare a Float16 capability.

Non-storage-related use of fp16 requires Float16 capability,
and shaderFloat16 Vulkan feature from VK_KHR_shader_float16_int8.
Such uses include declaring constants, using fp16 arithmetic, etc.

Components: Vulkan

Affects:
dEQP-VK.spirv_assembly.instruction.compute.float_controls.*

VK-GL-CTS issue: 1874

Change-Id: Ia403aac8ae32646329f67bf48d6231af233e9c20